In brief, the British magazine Autocar reports--rather breathlessly--that BMW is developing a "radical" new hybrid system that uses a standard Mini front-wheel-drive powertrain up front, plus a hydrogen-based system on the rear axle.

That system would use a small (5-kilowatt) hydrogen fuel cell, supplied by a tank of compressed hydrogen, to power an 82-kilowatt electric motor driving the rear wheels.

... and supercapacitors too!

Supercapacitors, which can quickly store and release power at very high voltages, would be used to buffer the fuel cell against the highest loads. The fuel cell and engine might work together under high load as well, though how that provides zero emissions is unclear.

Spy shots appear to show a BMW 1-Series hatchback serving as a test mule, apparently converted to front-wheel-drive with the hydrogen system at the rear. In the shots, a cryogenic tank is seemingly fueling the car through a port over the right rear wheel.

The goal is to let vehicles enter and travel through European city centers in emissions-free mode. In the coming decade, many cities are expected to adopt emissions-free zones--or tax emitting cars heavily--in the densest urban cores.

Carmakers obviously don't want their products banned altogether from such cores, so they're experimenting with many different ways to meet this need--among them, for instance, the Jaguar-Lotus LimoGreen serial hybrid program.

At the end of the article on the new hydrogen hybrid concept, Autocar notes discreetly:

The fuel-cell hybrid has not yet been given the green light by BMW bosses...the firm has to be certain that sufficient sales would be generated to ensure financial viability. It also needs to decide whether hydrogen fuel pumps will become available in most large cities.